Hot Dip Galvanized Flat Washers
Installing DTI's under an HDG flat washer will often not work correctly because the ASTM F436 flat washer can be very soft, in fact softer than the DTI. ASTM F436 currently allows the hardness of hot dipped flat washers to go as low as Rc 26 compared to Rc 38 to 45 for all other flat washers. All flat washers, whether hot dipped galvanized or not, should be made to a hardness of Rc 38 to 45, in order to function correctly for all structural purposes including to compress a DTI.
If flat washers softer than Rc 32 are found on a project, the DTI bumps simply dig into the soft HDG flat washer and therefore the apparent residual DTI gap will not correctly indicate tension. Also, if the flat washers are soft, the squirt feature may occur late.
The solution is to change the DTI position putting it under the bolt head, or replace the HDG flat washer with an MG flat washer. Alternatively, correctly made hot dipped flat washers to Rc minimum 38 can be purchased in the marketplace and are no more expensive.